There's an interesting story in the Denver Post that points out that the WTC bombing actually wasn't the first foreign attack on US mainland soil since the War of 1812, as many have mistakenly pointed out. I had forgotten about this myself, but the Japanese did send balloon bombs over during the war, and some of them did quite a bit of damage (this story points out that one made it at least as far east as Fort Collins, CO).

The real point of the story is that few were aware of it at the time (or even now) because the media made a conscious decision to not publicize it. As a result, the Japanese thought it was a flop and stopped wasting resources doing it.